The invention relates to a rotating electrical machine with an internal cooling system according to the features disclosed in the preamble to claim 1.
In rotating electrical machines, it is known to use contacting devices in order to electrically contact coils disposed in a rotating armature. These contacting devices have slide contacts which remain in sliding contact with a brush apparatus. The brushes, which are as a rule produced based on graphite materials, are acted on by a spring force which presses the brushes against the rotating contacting device. The contacting device here rotates, for example, at a speed of approximately 3,000 to 8,000 rpm. Both the slide contacts and the brushes experience wear during operation of the electrical machine. In order to minimize wear that is due to tribological conditions, it is known to add solid lubricants, binding agents, and/or cleansers to the graphite materials used for the brushes, in addition to the metal components already present.
In order to cool rotating electrical machines, on the one hand, systems are known in which an air current is conveyed over the components to be cooled and on the other hand, there are known embodiments in which a fluid cooling medium is conveyed past the components to the cooled. If a hermetically sealed housing is provided for noise protection reasons, or in order to protect the components of the electrical machine from external influences, then a heat flow from the inside of the housing can take place only through its walls. Such internal cooling systems have been produced up to this point only using a gas as the cooling medium. A disadvantage to this is the low thermal conductivity of the cooling medium so that only an insufficient heat dissipation can be expected. The use of a fluid cooling medium would lead to an undesirable increase in the shear stresses in the vicinity of rotating components so that an embodiment of this kind is only useful for electrical machines that operate at low speeds. Furthermore, when fluid coolants are used, there are additional tribological stresses in the vicinity of the slide contacts. Since the slide contacts are wetted or covered by the coolant, a penetration of oxygen is prevented. As a result, an oxidic protective layer, which covers the slide contacts of the contact device, wears away more quickly and the overall service life of the slide system is reduced.
By contrast with the prior art, in a rotating electrical machine with the features disclosed in claim 1, through the use of a special cooling medium, it is possible to keep the friction losses inside the electrical machine low and nevertheless to minimize the wear in the vicinity of the slide system. Since the electrical machine has an internal cooling system in which a fluid/gas mixture used as a cooling medium circulates around the components to be cooled, including the slide system, and since the slide contacts are comprised of an alloy with at least one alloy component that has an affinity for oxygen, an abrasion of the slide contacts of the contacting device due to tribological conditions during the sliding contact of the brushes against the slide contacts can be reduced and at the same time a very effective cooling of the components can be achieved.
The use of a fluid/gas mixture, preferably an oil, as a cooling medium causes the formation of an aerosol when the operation of the electrical machine is started. The aerosol has a greater thermal conductivity than a gas, but in terms of friction losses, has a more favorable viscosity than a cooling fluid. Depending on the application, a fluid component can make up 1 to 30% of the fluid/gas mixture. This fluid content and a selection of the fluid can be adapted to the existing requirements of the respective application. In spite of the aerosol formation, all of the surfaces of the components of the electrical machine disposed inside the housing are wetted. Since this also includes the vicinity of the slide system, in particular the slide contacts of the contacting device, the material selection indicated must prevent a premature wear.
In a preferred embodiment of the invention, the slide contacts are designed in such a way that a main component of the alloy is copper and the alloy component with the oxygen affinity is a base metal and/or a metalloid. It is clear from this that the selection of possible alloys is not limited to binary or tertiary systems, but permits any combination of the corresponding alloy components. In this connection, the alloy components with oxygen affinity are preferably added in a weight percentage of approximately 0.05 to 3%, in particular 0.3 to 0.9%.
It has turned out to be particularly advantageous to select the base metal as an element or a combination of elements from the group including magnesium, zirconium, titanium, hafnium, tungsten, molybdenum, vanadium, and iron. A particularly preferable metalloid can be an element or combination of elements selected from the group including tellurium, silicon, and boron. Moreover, it has turned out to be advantageous if the alloy contains companion elements such as oxygen and/or phosphorus in the parts per thousand or parts per million range.
Other preferred embodiments of the invention ensue from the remaining features disclosed in the dependent claims.